ORGANISATION AND STRUCTURE OF MUSCLE 



21 



distinctions, morphological as well as physiological, between the 

 several muscles vegetative and animal. This is admitted, in a 

 general sense, by the common division of vertebrate muscles into 

 two chief groups smooth and striated. The latter are technically 

 opposed as muscle-fibres, in a strict sense, on account of their 

 length, to muscle-ce^s with a single nucleus. 



The smooth muscles and striated cardiac muscle- 

 cells of Vertebrates are the natural continuation of 

 the uninucleated muscle-cells, smooth and striated, 

 of Invertebrates, inasmuch as they are mainly com- 

 posed of short, usually uninuclear, elements of a 

 more or less distinctly fibrillated structure, which, 

 viewed from the surface, appear for the most part 

 extended and spindle-shaped. As regards smooth 

 muscle-cells in particular, they are not infrequently 

 drawn out into fibres, without any consequent 

 increase of nuclei. 



In cross-section the contractile fibre-cells appear 

 either rounded (when solitary), or flattened by the 

 pressure of the tightly -crowded elements into a 

 polygon or band -shaped figure. The long, oval, 

 often "rod -like" nucleus always lies in the middle 

 of the cell, surrounded by a somewhat richer ac- 

 cumulation of formative plasma. Wherever it is 

 possible to recognise the fibrillated structure (and 

 this is by no means invariable), the fibrils appear as 

 a multitude of fine, smooth, cylindrical fibres, running 

 parallel to the long axis of the cells, and^as seen in 

 transverse section bedded in a seemingly homo- 

 geneous mass of sarcoplasm (Fig. 13). 



But while in nearly all the cases we have 

 been considering (of Invertebrates) the fibrils oc- 

 cupied only one part of the section, surrounding 

 the sarcoplasm in a ring or segment, in the smooth muscle 

 of Vertebrates the fibrils are, as a rule, distributed equally 

 over the entire surface. It is highly probable that fibrils must 

 also exist in the cases where it has so far been impossible to 

 detect them. Sometimes they are very obvious, and appear, e.g., 

 in the fibres of the frog's stomach Engelmann (12) in transverse 

 section, with a high power, as little circular dots or spaces, which 



FIG. 13. Central 

 part of isolated, 

 smooth muscle- 

 cell from Frog's 

 stomach. (En- 

 gelmann. ) f 



